The removal of water and CO.sub.2 from air by selective adsorption of these prior to fractionation of the air into its major components (O.sub.2 and N.sub.2) by either cryogenic or non-cryogenic separation is well known. When the adsorbent bed or beds become laden with sorbed water and CO.sub.2, the adsorbent is regenerated. Such adsorption of contaminants and regeneration of the adsorbent may be carried out by various known techniques which may be characterized as employing either temperature swing adsorption (TSA) or pressure swing adsorption (PSA). In the TSA mode of operation the atmospheric air to be treated is generally cooled down to below room temperature (as to about 40.degree. F. or 4.degree.-5.degree. C.) before being passed through the adsorbent bed(s) for removal of water and CO.sub.2. The adsorbent bed(s) is regenerated by passing a hot purge gas therethrough followed by cooling to ready the same for subsequent repeated use. Since the feed air is initially refrigerated and the adsorbent subsequently subjected to thermal regeneration, this mode of operation is energy intensive.
In non-cryogenic air separation plants, such as those employing the molten salt processes, the clear air obtained by pretreatment has to be reheated for further processing, which makes the use of TSA operation for the front end clean-up of the air rather expensive. In large cryogenic plants for air fractionation the overall energy consumption of the TSA operation can be reduced by utilization of systems for recovery of heat of compression of the air feed and recovery of refrigeration values from waste gas. The use of such recovery systems in small cryogenic plants is not justifiable because of associated capital costs. Accordingly, in such instances the energy consumption, of a TSA process is further increased.
Systems operating in the pressure swing mode (PSA) regeneration of adsorbent beds used in removal of moisture and CO.sub.2 from the feed air generally rely on vacuum application to rid the adsorbent of residually contained sorbate, some of these PSA operations may also cool the feed air to below room temperature before charging the same to the water/CO.sub.2 removal step.